1. Field of the Invention
The present invention relates to a method of manufacturing microalloyed structural steels by rolling in a CSP plant or compact strip production plant, wherein the cast slab strand is supplied divided into rolling lengths through an equalizing furnace to a multiple-stand CSP rolling train and is continuously rolled in the rolling train into hot-rolled wide strip, wherein the strip is cooled in a cooling section and is reeled into coils, and wherein, for achieving optimum mechanical properties, a controlled structure development by thermomechanical rolling is carried out as the thin slab travels through the CSP plant.
2. Description of the Related Art
EP-A-0368048 discloses the rolling of hot-rolled wide strip in a CSP plant, wherein continuously cast initial material, after being divided into rolling lengths, is conveyed through an equalizing furnace directly to the rolling mill. Used as the rolling mill is a multiple-stand mill in which the rolled lengths which have been raised to a temperature of 1100.degree. C. to 1130.degree. C. in the equalizing furnace are finish-rolled in successive work steps, wherein descaling is carried out between the work steps.
In order to achieve an improvement of the strength and the toughness properties and the corresponding substantial increase of the yield strength and the notch value of a rolled product of steel, EP-A-0413163 proposes to thermomechanically treat the rolling stock.
In contrast to a normalizing deformation in which the final deformation takes place in the range of the normal annealing temperature with complete recrystallization of the austenite, in the case of the thermomechanical deformation temperature ranges are maintained for a specified deformation rate in which the austenite does not recrystallize or does not significantly recrystallize.
A significant feature of the thermomechanical treatment is the utilization of the plastic deformation not only for manufacturing a defined product geometry, but also especially for adjusting a desired real structure and, thus, for ensuring defined material properties, wherein non-recrystallized austenite is subjected to the polymorphous gamma-alpha-deformation (in the normalizing deformation the austenite is already recrystallized).
Prior to deformation in a conventional rolling mill, conventional slabs when used in the cold state are subjected to the polymorphous transformations:
melt.fwdarw.ferrite (delta).fwdarw.austenite A.sub.1 (gamma).fwdarw.ferrite (alpha).fwdarw.austenite A.sub.2 (gamma) PA1 melt.fwdarw.ferrite (delta).fwdarw.austenite A.sub.1 (gamma) PA1 a) a specific mixed crystal strengthening by an increased silicon content and/or PA1 b) a complex mixed crystal strengthening by an increased content of copper, chromium, nickel. PA1 the mixed crystal strengthening is added to the step of precipitation hardening; this makes it possible to utilize the CSP process for achieving higher strength classes in the material group of ferretic/pearlitic structural steels; PA1 the mixed crystal strengthening takes place in such a way that, for example, due to the alloy element silicon, the strengthening remains essentially unaffected by the hot deformation; in other words, the strengthening does not lead, for example, to deformation-induced precipitation. Consequently, such a steel has a quieter behavior in the train, because it is strengthened to a lesser extent by the deformation itself; therefore, the steel is more easily manipulated by control technology.
while the following is true for the CSP technology:
with an increased oversaturation of the mixed crystal austenite and an increased precipitation potential for carbonitrides from the austenite.
In order to utilize the peculiarities of the structure development during thermomechanical rolling in CSP plants in an optimum manner, it has been proposed in prior U.S. patent application Ser. No. 09/095,338 filed Jun. 10, 1998, now U.S. Pat. No. 6,030,470 corresponding to German Patent Application 1972534.9-24, for adapting to the thermal prior history of the thin slabs introduced into the CSP rolling plant with a cast structure, to allow a complete recrystallization of the cast structure which starts at the thermomechanical first deformation, before a further deformation takes place. As a result of this measure, and by adjusting defined temperature and shape changing conditions, a controlled structure development is achieved in the rolling stock as it travels through the CSP plant and the thermomechanical deformation is adapted in an optimum manner to the specific process parameters of the CSP method with its specific prior thermal history.